Such lane-change assistants are meant to prevent the driver from changing to an adjacent lane if an overtaking vehicle is approaching from behind on this adjacent lane, so that there would be a collision risk or at least an obstruction of the overtaking vehicle. In most cases a rear-space radar is used as sensor system in such lane-change assistants, i.e., a radar sensor which points to the rear or obliquely to the rear and is able to measure the distances, relative velocities and azimuth angles of the located vehicles. The lateral offset of the vehicle is then able to be calculated from the distance and the azimuth angle, and a decision can be made as to the road lane on which the vehicle is traveling. Furthermore, using the distance and the relative velocity, it is calculated whether a change to the adjacent lane is possible without endangering the cars traveling behind. If the decision module determines that a lane change is impossible and, furthermore, if it can be seen, for instance based on the state of the turn signal indicator or based on the steering actions of the driver, that the driver intends to change lanes, then a warning is output to the driver via the driver interface. This warning may take the form of an optical, acoustic or haptic signal. Especially useful is the output of an optical signal by a display device which is integrated in the outside mirror of the vehicle.
In view of traffic safety and the acceptance of the system, the lane-change assistant should be designed in such a way that, for one, unnecessary false warnings are avoided, but, for another, a warning will be output with high reliability in an actual dangerous situation. For this it is necessary to make a reliable decision as to whether the located vehicle is indeed traveling on the immediately adjacent left neighboring lane (in countries using right-hand traffic). This decision is difficult because the position-finding signals of the sensor system include error tolerances. If no video system having an electronic image-evaluation device is available in the vehicle with whose aid the road markings on the pavement are directly detectable, then there is the additional problem that the precise width of the road lanes is not known and can only be estimated. As a result, there may be doubtful cases in which it is not clear whether the located vehicle is on the adjoining left lane but stays relatively to the right within this lane, or whether the vehicle is on the own lane and is traveling at a relatively wide left offset with respect to the own vehicle. Similar doubtful cases also exist in the assignment of the located vehicle to the immediately adjacent lane or the one-after-the-next lane. If the road curves, then this decision becomes even more complicated. To solve this latter problem, U.S. 2003/0025597 A1 proposes to record the history of the roadway path.